I have both an SX10 and an SXX10 and an SA10...
This is how i would rate them:
SPL: SXX, SX, SA10
SQL: SA10, SXX, SX
SQ: SA10, SX, SXX
unfortunately i used the same box for each sub, 1.3ft^3 port with 3"x15" aero port, So i think the SA10 was more at home. The price point of the SA10 is also very attractive.
Here you can see the SA10s in action, and then the RE SXX in actions:
2x SA10 on AUDIOPIPE APSM1300